The present invention relates to a high NA (numerical aperture) objective lens adapted to an optical pick-up of an optical disc apparatus that is capable of using a plurality of kinds of optical discs whose cover layers are different in thickness. Particularly, the present invention relates to the objective lens that has a diffractive lens structure formed on a surface of a refractive lens.
The optical disc includes an information layer on which digital information is recorded, and a transparent cover layer that covers the information layer.
There are several types of the optical discs. A compact disc (CD) or a CD-recordable (CD-R) has the cover layer whose thickness is 1.2 mm, and the thickness of the cover layer of a digital versatile disc (DVD) is 0.6 mm.
Such a difference of thickness of the cover layer changes the relative position of the information layer with respect to a turntable, i.e., the distance between the optical pick-up and the information layer. Namely, the thicker the cover layer is, the greater the distance to the information layer from the optical pick-up is. For example, the optical pick-up is required to move a beam spot away from the optical pick-up by 0.6 mm in the cover layer, which is equivalent to 0.4 mm in air, when the DVD is replaced with a CD or a CD-R.
Although the position of a paraxial beam waist moves as the objective lens is moved, the change in the thickness of the cover layer changes a spherical aberration. If only the objective lens is moved in the optical pick-up when the disc is replaced with the different-thickness one, a wavefront aberration of the laser beam becomes large. For instance, when the objective lens, which is designed to minimize the spherical aberration for a DVD, is applied for reproducing the information from a CD, the spherical aberration becomes too large to reproduce the information even if the objective lens moves to place the beam spot on the information layer of the CD.
Further, the recording density of a DVD is higher than that of a CD, which requires the optical pick-up for a DVD to form a smaller beam spot than the optical pick-up designed for the exclusive use for a CD (hereinafter referred to as an exclusive CD pick-up). Since the diameter of the beam spot has a positive correlation with the wavelength of the laser beam, the optical pick-up for a DVD requires the laser source whose emission wavelength is 635 through 660 nm that is shorter than the emission wavelength of an exclusive CD pick-up (i.e., 780 through 830 nm). On the other hand, the reflection characteristics of a CD-R require the laser source whose emission wavelength is longer than 780 nm.
Accordingly, at least two laser sources are required for the optical pick-up to use a DVD and a CD-R.
It has been known as a prior art to form a diffractive lens structure on a surface of an objective lens to compensate a change of a spherical aberration. The diffractive lens structure has such a wavelength dependence that both of a spherical aberration at a short wavelength with a DVD and a spherical aberration at a long wavelength with a CD-R are corrected.
However, since the diffractive lens structure has the wavelength dependence, a wavefront aberration becomes large when the emission wavelength of the semiconductor laser becomes different from a design wavelength due to temperature change or to individual differences of semiconductor lasers. Such a wavefront aberration causes no problem for an optical disc having low recording density such as a CD or a CD-R, while it may interfere with the recording/reproducing of information data on/from an optical disc having high recording density such as a DVD because of low tolerance of wavefront aberration.
It is therefore an object of the present invention to provide an objective lens for an optical pick-up, which has a diffractive lens structure for correcting spherical aberrations in a plurality of kinds of optical discs having cover layers of different thickness, that is able to form a beam spot that has an appropriate size for respective optical discs even if the emission wavelength of a laser source becomes different from a design wavelength due to temperature change or to individual differences of the laser sources.
For the above object, according to the present invention, there is provided an improved objective lens for an optical pick-up which includes a refractive lens, and a diffractive lens structure having a plurality of concentric ring areas having minute steps at the boundaries therebetween and is formed on at least one lens surface of the refractive lens. The lens surface is divided into a high NA exclusive area through which a light beam of a high NA, which is necessary only for an optical disc having first recording density, passes, and a common area through which a light beam of a low NA, which is necessary and sufficient for an optical disc having low recording density, passes. The light beam of the high NA has a short wavelength and the light beam of the low NA has a long wavelength. The diffractive lens structure formed in the common area has a wavelength dependence of a spherical aberration, i.e., changes a spherical aberration thereof with a wavelength, such that the light beam of the short wavelength forms an appropriate wavefront for a thin cover type optical disc and the light beam of the long wavelength forms an appropriate wavefront for a thick cover type optical disc. On the other hand, the diffractive lens structure formed in the high NA exclusive area has a smaller wavelength dependence of a spherical aberration than that formed in the common area, and is designed such that a spherical aberration for the thin cover type optical disc at the short wavelength is adequately corrected.
With this construction, when the thin cover type optical disc is applied, the laser beams at the short wavelength passing through both of the common area and the high NA exclusive area are converged onto the information layer of the thin cover type optical disc because the diffractive lens structure adequately corrects a spherical aberration. On the other hand, when the thick cover type optical disc is applied, while the laser beam at the long wavelength passing through the common area is converged onto the information layer of the thick cover type optical disc because the diffractive lens structure in the area corrects the spherical aberration, the laser beam of the long wavelength passing through the high NA exclusive area is diffused because the diffractive lens structure in the high NA exclusive area does not adequately correct a spherical aberration at the long wavelength.
An additional optical path length added by the diffractive lens structure formed in the common area is expressed by the following optical path difference function "PHgr"C(h):
"PHgr"C(h)=(P2Ch2+P4Ch4+P6Ch6+. . . )xc3x97mxc3x97xcex
where P2C, P4C and P6C are coefficients of second, fourth and sixth orders, h is a height from the optical axis, m is a diffraction order and xcex is a working wavelength. Further, an additional optical path length added by the diffractive lens structure formed in the high NA exclusive area is expressed by the following optical path difference function "PHgr"E(h):
"PHgr"E(h)=(P2Eh2+P4Eh4+P6Eh6+. . . )xc3x97mxc3x97xcex
where P2E, P4E and P6E are coefficients of second, fourth and sixth orders. In such an expression, negative values of the second order coefficients P2C, P2E represent positive paraxial powers of the diffractive lens structure. Further, when the fourth order coefficients P4C, P4E are smaller than zero, the positive power increases with the distance from the optical axis, which presents an undercorrected spherical aberration.
It is preferable that the diffractive lens structure satisfies the following conditions (1) and (2);
P2C greater than P2Exe2x80x83xe2x80x83(1)
P4C less than P4E, P4C less than 0.xe2x80x83xe2x80x83(2)
Further, the width of the innermost ring area of the high NA exclusive area may be larger than the width of the outermost ring area of the common area.
Still further, when the refractive lens is made of plastic, the diffractive lens structure preferably satisfies the condition (3):
P4C less than P4E less than 0.xe2x80x83xe2x80x83(3)
In particular, it is preferable that the diffractive lens structure formed in the high NA exclusive area has such a wavelength dependence that a change of a spherical aberration due to a variation of the refractive index of the refractive lens with a temperature change is compensated by a variation of emission wavelength of a semiconductor laser with the temperature change.
Incidentally, when the diffractive lens structure has the above described functions, an inner edge of a ring area is protruded with respect to an outer edge of the adjacent inner ring area in every pair of the inner and outer ring areas within the common area and the high NA exclusive area. Further, when the optical path difference function "PHgr"E(h) of the high NA exclusive area is determined to make the absolute value of P4E small and the absolute value of P2E large while keeping the continuity of the optical path difference function "PHgr"C(h) of the common area, the outer edge of the outermost ring area of the common area is protruded with respect to the inner edge of the innermost ring area of the high NA exclusive area.
However, when the orientation of the minute step at the boundary between the common and high NA exclusive area is opposite to that at the other boundaries, the diffractive lens structure is hard to manufacture. Therefore, it is preferable that the innermost ring area is formed as an extra-wide ring area where the difference between the values of the optical path difference function "PHgr"E(h) at the inner edge and that at the outer edge is larger than one wavelength long. In this case, the orientations of the minute steps at all of boundaries become identical.